asp.net dropdownlist未从新创建的html下拉列表更新

我想通过使用css和javascript将asp下拉列表默认设置更改为富外观UI设计。更改后，未选择“原始设计”下拉列表。下面是js脚本。给定脚本未更新原始asp.net淹没选定值

<script>

    var x, i, j, selElmnt, a, b, c;
    /*look for any elements with the class "custom-select":*/
    x = document.getElementsByClassName("custom-select");
    for (i = 0; i < x.length; i++) {
      selElmnt = x[i].getElementsByTagName("select")[0];
      /*for each element, create a new DIV that will act as the selected item:*/
      a = document.createElement("DIV");
      a.setAttribute("class", "select-selected");
      a.innerHTML = selElmnt.options[selElmnt.selectedIndex].innerHTML;
      x[i].appendChild(a);
      /*for each element, create a new DIV that will contain the option list:*/
      b = document.createElement("DIV");
      b.setAttribute("class", "select-items select-hide");
      for (j = 1; j < selElmnt.length; j++) {
        /*for each option in the original select element,
        create a new DIV that will act as an option item:*/
        c = document.createElement("DIV");
        c.innerHTML = selElmnt.options[j].innerHTML;
        c.addEventListener("click", function(e) {
            /*when an item is clicked, update the original select box,
            and the selected item:*/
            var y, i, k, s, h;
            s = this.parentNode.parentNode.getElementsByTagName("select")[0];
            h = this.parentNode.previousSibling;
            for (i = 0; i < s.length; i++) {
              if (s.options[i].innerHTML == this.innerHTML) {
                s.selectedIndex = i;
                h.innerHTML = this.innerHTML;
                y = this.parentNode.getElementsByClassName("same-as-selected");
                for (k = 0; k < y.length; k++) {
                  y[k].removeAttribute("class");
                }
                this.setAttribute("class", "same-as-selected");
                break;
              }
            }
            h.click();
        });
        b.appendChild(c);
      }
      x[i].appendChild(b);
      a.addEventListener("click", function(e) {
          /*when the select box is clicked, close any other select boxes,
          and open/close the current select box:*/
          e.stopPropagation();
          closeAllSelect(this);
          this.nextSibling.classList.toggle("select-hide");
          this.classList.toggle("select-arrow-active");
        });
    }
    function closeAllSelect(elmnt) {
      /*a function that will close all select boxes in the document,
      except the current select box:*/
      var x, y, i, arrNo = [];
      x = document.getElementsByClassName("select-items");
      y = document.getElementsByClassName("select-selected");
      for (i = 0; i < y.length; i++) {
        if (elmnt == y[i]) {
          arrNo.push(i)
        } else {
          y[i].classList.remove("select-arrow-active");
        }
      }
      for (i = 0; i < x.length; i++) {
        if (arrNo.indexOf(i)) {
          x[i].classList.add("select-hide");
        }
      }
    }
    /*if the user clicks anywhere outside the select box,
    then close all select boxes:*/
    document.addEventListener("click", closeAllSelect);
    </script>

变量x，i，j，sellemnt，a，b，c；
/*查找类为“custom select”的任何元素：*/
x=document.getElementsByClassName（“自定义选择”）；
对于（i=0；i

Html代码：-

    <div class="selectBox " >
                                       <div class="input-group mb-2">
                                           <div class="new-select-box d-flex">
                                                   <div class="custom-select-new" id="db"  style="width:400px;">
                                                       <label for="exampleFormControlSelect1">Operation</label>
   <select name="test" onchange="javascript:setTimeout(&#39;__doPostBack(\&#39;test\&#39;,\&#39;\&#39;)&#39;, 0)" id="test" style="font-family:Calibri;">
       <option selected="selected" value="">Select value...</option>
       <option value="A">A</option>
       <option value="b">b</option>
       <option value="c">c</option>
   </select>

活动
选择值。。。
A.
B
C

此脚本将在遇到时运行-仅当

元素位于文档的最底部，或者如果它被移动到

函数中，该函数通过
延迟执行，或者在
DOMContentLoaded
事件之后执行，它才会起作用。此外，除非您发布呈现的HTML，否则我们无法为您提供任何帮助（不是你的
.aspx
或
.cshtml
源代码）。你确定你的
元素有
class=“自定义选择”吗
set，例如？您发布的脚本看起来像是在2000年代中期编写的，因为它使用了
getElementsByClassName
和
getElementsByTagName
而不是更现代的
querySelector
和
queryselectoral
。我怀疑您根本没有编写这个脚本，而是复制了它如果你真的要通读脚本，那么也许你会明白为什么它不起作用。你的好奇心在哪里？这是html呈现代码。